From 4b9fbdb5a713745dfdb13042e33ba2345e6860e1 Mon Sep 17 00:00:00 2001 From: Eli Zaretskii <eliz@gnu.org> Date: Tue, 19 May 2020 18:46:27 +0300 Subject: [PATCH] ; Update TODO item about ligature support * etc/TODO: Add the todo item for moving cursor "inside" a ligature. --- etc/TODO | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) diff --git a/etc/TODO b/etc/TODO index 20262a77e97..f983fa27d33 100644 --- a/etc/TODO +++ b/etc/TODO @@ -264,6 +264,37 @@ of the mode line. The prettify-symbols-mode should be deprecated once ligature support is in place. +A related, but somewhat independent, feature is being able to move the +cursor "into a ligature", whereby cursor motion commands shows some +pseudo-cursor on some part of a ligature. For example, if "ffi" is +displayed as a ligature, then moving by one buffer position should +show the middle part of the ligature's glyph similar to the cursor +display: some special background and perhaps also a special +foreground. There are two possible ways of figuring out the offset at +which to display the pseudo-cursor: + + . Arbitrarily divide the ligature's glyph width W into N parts, + where N is the number of codepoints composed into the ligature, then + move that pseudo-cursor by W/N pixels each time a cursor-motion + command is invoked; + . Use the font information. For example, HarfBuzz has the + hb_ot_layout_get_ligature_carets API for that purpose. However, + it could be that few fonts actually have that information recorded + in them, in which case the previous heuristics will be needed as + fallback. + +One subtle issue needs to be resolved to have this feature of +"sub-glyph" cursor movement inside composed characters. The way Emacs +currently displays the default block cursor is by simply redrawing the +glyph at point in reverse video. So Emacs currently doesn't have a +way of displaying a cursor that "covers" only part of a glyph. To +make this happen, the display code will probably need to be changed to +draw the cursor as part of drawing the foreground and/or background of +the corresponding glyph, which is against the current flow of the +display code: it generally first completely draws the background and +foreground of the entire text that needs to be redrawn, and only then +draws the cursor where it should be placed. + ** Support for Stylistic Sets This will allow using "alternate glyphs" supported by modern fonts. For an overview of this feature, see -- 2.39.5